1. The Field of the Invention
The present invention relates to a medical device. More particularly, the present invention relates to a flexible stent that provides elevated torque-absorbing properties.
2. The Relevant Technology
Atherosclerosis, sometimes called the hardening or clogging of the arteries, is an accumulation of cholesterol and fatty deposits, called plaque, on the inner walls of the arteries. Atherosclerosis causes a partial or total blockage of the arteries and, consequently, a reduced blood flow to the heart, legs, kidneys, or brain.
Traditionally, clogged arteries have been treated with surgical procedures that involve the removal of the diseased arterial tract. Angioplasty procedures, during which a stent is inserted in the diseased portion of the artery, have gained increased acceptance during the last two decades because of the reduced complexity of this procedure in comparison with other surgical procedures and because of the consequent reduction in risk and discomfort to the patient.
Referring first to FIG. 1, a stent 20 is a small tubular element that typically has a cylindrical structure 22 and that, once placed within a blocked vessel, acts as a scaffold that keeps the vessel open. Stent 20 may be implanted in a bodily vessel by disposing the stent over a balloon tipped catheter, by driving the stent to a target location in a vessel, and by subsequently inflating the balloon at the target location.
Alternatively, stent 20 may be caused to self-expand without the use of a balloon by manufacturing stent 20 from a shape memory material and by disposing stent 20 over a catheter in a contracted delivery configuration. Stent 20 is successively driven to a target location in a vessel, where a sheath covering stent 20 is withdrawn and stent 20 is allowed to self-expand. One type of self-expanding stent is produced from a superelastic material and is compressed inside the sheath into a contracted delivery configuration. When the stent is released from the sheath, the flexible material causes the stent to spring back to its original shape and size before compression. Another type of self-expanding stent is produced from a thermo-elastic shape-memory material that is formed into a desired size and shape and is then annealed at a temperature higher than a transition temperature. After cooling the stent to a temperature below the transition temperature, the stent becomes soft and can be reduced to a smaller size by crimping, so that it can be delivered to the target location, where the stent is warmed to a temperature above the transition temperature and returns to the preprogrammed size and shape. The present invention relates to both to balloon expandable stents and to self-expanding stents, as explained in greater detail in the following sections.
The stents in the prior art are formed as a metal mesh or, in general, as a web structure that provides some degree of flexibility. Certain types of anatomies require that stents with elevated degrees of flexibility be employed, for example, stents to be implanted in the carotid artery, because the bifurcated anatomy of the carotid artery and frequent movements of that part of the body require a stent that can adapt to such anatomy. The stent designs in the prior art typically increase flexibility by increasing cells size in the mesh or in the web structure. Therefore, whenever stent flexibility is increased in the stents in the prior art, scaffolding support is affected negatively due to the related reduction in web density.
A prior art stent is disclosed in U.S. Pat. No. 5,104,404, which teaches an articulated stent in which stent segments, formed by diamond-shaped cells disposed in ring form, are connected one to the other at some but not all of the tips of the diamond-shaped cells. This arrangement provides for a stent with a high degree of longitudinal flexibility, but also for limited support to the arterial walls at the junctions areas between the different stent segments.
With reference now to FIG. 2, other designs in the prior art have attempted to increase stent flexibility by forming the stent as a plurality of web rings 24 that are disposed longitudinally along tubular body 22 and that are coupled one to the other by flexible connectors 26. Designs of this type are disclosed in U.S. patent application Ser. No. 10/743,857, U.S. Pat. Nos. 6,682,554 and 6,602,285, International Application PCT/EP99/06456, and German Patent Application Serial No. 19840645.2, the entireties of which are incorporated herein by reference. The function of flexible connectors 26 is to facilitate the bending of stent 20 by creating longitudinal segments softer than web rings 24. At the same time, flexible connectors 26 transmit torque from one web ring 24 to adjacent web rings 24 when a bending force is applied to tubular body 22 or when a radial force is applied to tubular body 22, for example, during deployment of stent 22 from the contracted delivery configuration to the expanded delivery configuration. Such a transmission of torque may cause different web rings 24 to rotate differentially upon application of a bending force or upon deployment of stent 20.
One example of stent construction based on longitudinally alternating of web rings coupled by flexible connectors can be found in U.S. Pat. No. 6,190,403, which discloses a stent having a plurality of web rings disposed in longitudinal order. Each of the web rings is formed by longitudinally-oriented cells disposed circumferentially and is joined to a neighboring web ring by sinusoidal connectors that couple cell tips that are longitudinally aligned one with the other. The stent of the '403 patent provides an elevated degree of scaffolding to the arterial walls, though its structure provides only for a limited degree of longitudinal flexibility due to the limited extent of longitudinal translation that is possible between web rings when a compressive force is applied.
Another example of stent construction based on longitudinally alternating web rings coupled by flexible connectors can be found in U.S. Pat. No. 6,451,049, which discloses a stent having a plurality of waveform rings coupled by longitudinal connectors that include a “U” bend. This construction also provides for an elevated degree of scaffolding of the vessel walls, but its flexibility is constrained by the limited ability to compress of the flexible segments.
In order to increase stent flexibility, stent designs have been introduced in which the flexible connectors between web rings do not have a longitudinal orientation but instead have a transverse orientation. Examples of such stent designs can be found in U.S. Pat. Nos. 5,980,552; 6,059,811; 6,508,834; and 6,589,276. The transverse orientation of the flexible connectors induce the web rings to rotate one in relation to the other upon the application of a bending or radial force to the stent, and in order to reduce torsional stress in the stent during bending and during expansion, the flexible connectors may have alternating directions. For example, the flexible connectors connecting two neighboring rings may be oriented in a direction opposite to the direction of the next set of flexible connectors. If the web rings are prevented from rotating, the torsional stress in the stent becomes absorbed by the flexible connectors and by the web rings, possibly causing the connectors to warp along their entire length. Additionally, this type of construction causes a foreshortening of the stent during expansion.
This problem is illustrated in greater detail in FIGS. 3A-3B. A typical connector 28 couples first web ring 30 to second web ring 32 by connecting junction bend 34 on first ring 30 to junction bend 36 on second ring 32. In order for stent 38 to provide an elevated degree of scaffolding to the vessel within which stent 38 is implanted, each junction bend 34 on web ring 30 is coupled to a junction bend 36 in web ring 32, increasing stent density. The higher the density of stent 38, however, the lower the flexibility, which may be increased by increasing the length of connector 28.
When the length of connector 28 is increased, the bending capability and the flexibility of stent 38 is increased correspondingly because the moment applied by connector 28 to web rings 30 and 32 upon the application of a bending force on stent 34 is increased correspondingly. Unfortunately, long connectors disposed transversally on stent 38 can extend along a significant amount of the outer circumference of stent 38. For example, if stent 38 has a diameter of 1.6 mm and if connector 28 is one mm long, connector 28 extends for approximately 72 degrees along the circumference of stent 38. In turn, long connectors 28 will exert a significant amount of torque on junction bends 34 and 36, and, consequently, on web rings 30 and 32, and may warp along their entire length. In addition, long connectors 28 cause the size of stent cells to increase during expansion, therefore, long connectors cause a reduction in the scaffolding properties of stent 20, or a reduction in the ability of stent 20 to effectively support the vessel, into which stent 20 is implanted.
By having long connectors 28 disposed in a direction essentially perpendicular to the longitudinal axis of stent 20, connector 28 also tend to retain the bend radius of stent 20 during expansion and to cause a distortion of stent 20 in the expanded configuration.
Attempts have been made in the prior art to provide long connectors that extend along relatively limited portions of the circumference of stent 38 and that increase vessel support. For example, U.S. Pat. Nos. 5,449,373; 6,203,569; 6,740,114; 6,790,227; 6,942,689; 6,955,686; 6,998,060; 6,679,911; and 6,875,228 disclose stent constructions, in which the connectors have a variety of shapes in the form of the letters “M”, “N”, “W” or similar shapes, but which all include a plurality of segments oriented at certain angles with respect to the longitudinal axis of the stent. In particular, each of the prior art designs contains one or more central segments that are oriented at an angle with respect to the longitudinal axis of the stent, causing rotations in different degrees upon the application of a torsional force on the connector, for example due to a bending of the stent or during expansion.
Therefore, it would be desirable to provide a stent that generates an elevated degree of scaffolding to a bodily vessel while remaining highly flexible.
It would also be desirable to provide a stent, in which long connectors can be employed to increase stent flexibility and that can absorb torsional forces applied to the stent without warping along their entire lengths.